NOT ALL SPIDERS BUILD WEBS TO CAPTURE PREY

by Whit Gibbons

July 5, 2009

Part of the intrigue of ecology and animal behavior is that the natural world is full of exceptions. For example, when we think of spiders, we think of spider webs. But not all spiders rely on spinning a web to capture their prey. Several species of jumping spiders, known scientifically as Portia, have a different strategy for obtaining a meal. Stealth, deceit, and fierceness are all part of the package with these little predators.

Most spiders eat mainly insects. But Portia jumping spiders rely primarily on other spiders as a food source. And they capture their prey by pouncing on them. The tactics they use to outwit and capture web-building spiders, as well as members of their own species, sound like something out of a science fiction movie.

The Portia's strategy includes a deceptive appearance and a cunning attack plan. Part of their tactical design is to be undetectable, yet they have excellent visual abilities themselves, plus fangs and venom. With four pairs of keen eyes that sense motion, they have better eyesight than most spiders. A dark brown color and hairy-looking legs give Portia the guise of a piece of debris that can go unnoticed on another spider's web.

As if being inconspicuous were not in itself sufficient for Portia to enter the lair and attack other spiders, they extend their behavioral antics a step further. They actually lure web-building spiders by mimicking trapped insects. Many spiders stay in a protected area at the base of their web and monitor vibrations. An insect trapped in a web makes predictable movements while trying to escape. The spider knows it has captured a particular prey item, like a fly or grasshopper.

Stealthily, Portia enters another spider's web, not only not getting stuck but also not making vibrations that are detectable as anything more than that of a breeze or falling leaf. The tiny predator then creates vibrations that mimic an entangled insect. As the web-building spider emerges from its hiding place intent on making a meal of a trapped victim, the tables are unexpectedly turned. The would-be predator, unprepared for an attack, can become the prey. By assuming it has a home field advantage against a defenseless insect, the web-builder may be caught off guard and killed by the Portia.

But a successful outcome is not always assured for the little invaders. Most spiders have fangs, and spiders intended as victims are often much larger than Portia. If a spider moves in quickly, Portia may be killed before it has an opportunity to attack. Hence, these small warriors of the spider world sometimes use other tactics.

The Portia may employ camouflage, by looking like a piece of trash. When the larger spider approaches, Portia may change its drumming pattern on the web in a manner that belies its presence. For example, it may make the web move as it would in a gentle breeze, so the bigger spider does not attack immediately, perceiving that only a piece of debris has fallen into the web. Awaiting the right moment, Portia can ambush effectively by catching the other spider completely unawares.

These canny invaders do not limit themselves to eating other species of spiders. Portia spiders, especially females, will also eat their own kind. Sometimes when females encounter each other they will fight until one kills the other.

Female Portia spiders also eat males, especially during the mating season. Both adult and juvenile females on occasion release chemical signals that attract males--as meals, not mates. The ploy works because a signaling female may actually have mating in mind. If a female Portia spider kills and eats a male before mating with him, it indicates that she has rejected him as a mate but has accepted him as a meal. The species of Portia spiders persist because sometimes the females wait until after they mate before they turn on the male and devour him. Mate- or meal-luring behavior is the norm for these spider-eating spiders. Apparently, the odds are good enough to keep attracting males, a phenomenon some might consider to be universal among all species of animals.

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